The present invention relates to a method for monitoring a pressure sensor in a pressure-controlled delivery and metering system for a liquid reactant of a catalytic converter, in particular of an SCR catalytic converter, and to a corresponding delivery and metering system.
Methods and devices are known for operating an internal combustion engine, in particular in motor vehicles, in the exhaust gas region of which an SCR (Selective Catalytic Reduction) catalytic converter is arranged, which catalytic converter reduces the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. As a result, the proportion of nitrogen oxides in the exhaust gas can be considerably reduced. For the reaction to proceed, ammonia (NH3) is required as a reducing agent. This reducing agent or reactant is mixed into the exhaust gas. In order to make available ammonia, an aqueous urea solution is usually used, said solution being injected into the exhaust gas section downstream of the SCR catalytic converter using a metering device. The injection of the urea solution into the exhaust section is carried out as a function of demand and should take place very precisely.
The urea solution is usually stored in the motor vehicle in a urea solution tank. In general, a delivery and metering system is provided for delivering the urea solution from the tank. This system comprises a delivery unit with a delivery pump and a pressure line which opens into a metering unit via which the reactant is injected under pressure into the exhaust section. The system is a pressure-control unit which is controlled and monitored substantially on the basis of the signals of a pressure sensor which is assigned to the pressure line. The spraying in of the reactant by means of the metering unit is carried out at a predefineable system pressure.
Since the delivery and metering system is a component of the motor vehicle which is relevant for exhaust gas, it is necessary to monitor the functioning of this system. In current systems, the pressure sensor is monitored only statically or for minimum and maximum limits. As a rule, an offset is checked when a system start occurs. During the increase of pressure in the system and during the metering mode it is as a rule checked only whether the pressure is in the expected range. If an overpressure fault occurs, it is therefore not possible to determine precisely whether, for example, a line is blocked or whether a defective pressure sensor is present. Since as a rule there is no second pressure sensor in the system, dynamic plausibility checking of the pressure sensor is not possible.